This invention relates generally to fluid-activated, percussive, down-the-hole drills, and more particularly to such drills that use compressed air as the percussive fluid for removal of debris from the drillhole, after the compressed air actuates the drill and is exhausted out the drill.
Percussive drills of this type comprise a hollow cylindrical drill casing; a chuck connected to a bottom end of the drill casing; a drill bit connected to the chuck; a back head assembly fluidly connecting the drill to a drill string; a piston slidably mounted within the drill casing for reciprocating between an impact position in contact with the drill bit and a return position not in contact with the drill bit; drive chamber fluid passageway means for providing a first reservoir of percussive fluid for reciprocating the piston between the return position and the impact position; return chamber fluid passageway means for providing a second reservoir of percussive fluid when the piston is in the return position, for exhausting from the drill; and exhaust fluid passageway means for conducting the percussive fluid from the return chamber fluid passageway through the drill to an exhaust port, to lift debris up a drillhole.
For shallow hole drilling, that is for depths less than about 600 feet, a conventional volume of compressed air that is exhausted at the completion of each cycle of the piston is sufficient to lift the air column and debris in the drillhole. However, for deep hole drilling, that is for depths above 600 feet, a greater amount of compressed air must be expelled by the drill for each cycle of the piston, due to the increased weight of air and debris in the drillhole.
In order to increase the volume of compressed air used for each stroke of the piston, it is possible to increase either the volume of the drive chamber or the volume of the return chamber, or both. The drive chamber volume is dictated by the diameters of the drill casing, the piston and by the piston's stroke length, and changing these parameters is difficult, without rebuilding the drill. This leaves the return chamber volume for adjustment. Prior art adjustment of the return chamber volume for deep hole and shallow hole drilling is achieved by inserting a different bearing between the drill casing and the drill bit, for each volume of return chamber desired.
This requires special bearings for each application, and can lead to confusion by the operator as to which bearing is to be used for a specific application.
The foregoing illustrates limitations known to exist in present percussive down-the-hole drills. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.